FileManager: Estimate transfer time

Use the total bytes transferred count to estimate the time left
for the copy operation to finish.  With the estimate label, the
two progress bars were deemed superfluous, so the only remaining
progress bar is the overall copy progress, that is updated more
frequently.  (The same progress is also shown in the task bar,
so you can minimize the window and still be informed of the
progress.)

Userland/Applications/FileManager/FileOperationProgressWidget.h

@@ -26,6 +26,7 @@
 
 #pragma once
 
+#include <LibCore/ElapsedTimer.h>
 #include <LibGUI/Widget.h>
 
 namespace FileManager {
@@ -45,6 +46,9 @@ private:
 
     void close_pipe();
 
+    String estimate_time(off_t bytes_done, off_t total_byte_count);
+    Core::ElapsedTimer m_elapsed_timer;
+
     RefPtr<Core::Notifier> m_notifier;
     RefPtr<Core::File> m_helper_pipe;
 };